The present invention is related generally to weighing machines, and more particularly to apparatus and methods for controlling vibratory feeders of weighing machines.
Weighing machines incorporating vibratory feeders are well-known for separating bulk product such as peanuts, candy, fruit, frozen chicken parts and other products into discrete amounts for subsequent packaging. These net weight machines are of two, general types, single bucket and combination weighing machines.
A typical net weight weighing machine includes one or more vibratory feeders arranged serially, that is up- or downstream relative to one another. By using more than one than one feeder, the bulk product is more discretely separated and distributed for weighing. A typical vibratory feeder includes a pan with a feed end into which product is loaded, an opposite or "discharge" end, and a mechanism for vibrating the pan. During a weighing cycle, bulk product to be weighed is loaded into a pan from a hopper assembly, a conveyor belt or from a preceding pan, and moves toward the discharge end as the pan vibrates. Discharged product falls into a downstream pan, or into the weigh bucket in the case of the pan immediately preceding the bucket or other accumulating device, and the product in the bucket is weighed. When a predetermined weight of product has been discharged into the bucket, the contents of the bucket are dumped into a discharge chute to be packaged in individual packages or containers.
A combination weighing machine typically includes vibratory feeders that operate in the same manner, but include a number of feeders, for example 10-14 feeders and respective buckets. Bulk product is selectively loaded from a central vibratory feeder and into each feeder, or each series of feeders. Then, the product moves toward a respective discharge end of each pan, and is discharged into a respective bucket, at which point the weight of product in each bucket is measured. Preferably, each bucket contains between one-fifth and one-third of the target weight of product. A processor scans the measured weights in the buckets, selects the combination of buckets whose total weight most nearly matches a target weight, and causes the emptying of the selected buckets into a discharge chute to be packaged.
In both machines, each of the vibrators for vibrating the pans vibrates at an amplitude of vibration in response to an input signal, such as is provided by an AC line. The vibrators are operated, and the pans are vibrated, over the course of a feed cycle in which a target weight of product is to be discharged into a weigh bucket. Each weighing cycle begins with an empty bucket. In a single bucket weighing machine, the vibrators are initially operated at a preselected voltage to produce a "bulk feed" rate for a first time period, and product is discharged from each pan until a first or "bulk set point" weight in the buckets is reached. The voltage is then reduced to provide a reduced amplitude of vibration and the feed rate, which is continued for a second time period. Alternatively, if the pan is a "split pan" type including a bulk feed trough and a dribble feed trough, an associated cut off is activated to stop discharge of product from the bulk feed trough during dribble feed. In a combination weighing machine, each vibrator is operated at a preselected voltage for the entire feed cycle. As the weight of discharged product in the bucket reaches the target weight, the discharge of further product is stopped either by stopping the vibrator or alternatively by activating a cutoff for the dribble feed trough. A typical machine performs between about 15 and 60 feed cycles per minute. Accordingly, each tenth or other portion of a second that is saved results in substantially improved performance, for example by decreasing the amplitude and increasing the duration of dribble feed to more accurately discharge product or by increasing the cycles per minute of the machine.
In order to optimize the efficiency of these weighing machines, it is essential that the amplitude of vibration produced in a pan be identical from cycle to cycle, especially during dribble feed, and that the depth of product in a pan be as consistent as possible from cycle to cycle. These criteria are especially important in a single bucket system in which the target weight must be discharged in every bucket and after every cycle. With consistent product depth and amplitude of vibration, it is easier to achieve predictable and reliable feed rates and resulting target weights from cycle to cycle, since the vibrators can be operated at the bulk and dribble feed rates for substantially predetermined time periods.
Ideally, a given input voltage produces a corresponding amplitude of vibration, and thus a corresponding feed rate. However, identical input voltages tend to produce different amplitudes of vibration among vibrators, even among vibrators of the same make and model. Moreover, those familiar with the operation of voltage-sensitive equipment will recognize that the input voltage from a power line typically varies by as much as ten volts over the course of a single day, and it is therefore extremely difficult to repeatably provide a given input voltage over an entire day. The variations in input voltage produce drastic variations in amplitude of vibration of the vibrator and pan, and thus in feed rates of product from cycle to cycle and over the course of a day.
Previous attempts to control amplitude of vibration in a pan have operated in an "open loop" manner, that is without any consideration for the actual amplitude of vibration of a pan. In an operator adjusted system, an adjustable transformer is employed to alter the AC voltage applied to the vibrator. In another system, a microprocessor operates a phase-controlled, voltage-controlled relay. When less than a full voltage is applied to the relay, less than a full amplitude is produced by the vibrator. These systems do not operate in a linear manner, so that an adjustment to the transformer does not produce a proportionate adjustment to the amplitude of vibration of the vibrator.
Open loop controls have a number of shortcomings. As there is no provision for detecting and adjusting the input voltage in response to variations in the AC line voltage and as the above-noted ten-volt variation can result in drastically varied feed rates over the course of a day, it is virtually impossible to provide repeatability of a feed cycle from one cycle to the next. There is likewise no provision to compensate for aging during the useful life of the springs of each vibrator. Consequently, a given voltage applied to a particular vibrator at different times over a period of several months will produce different amplitudes of vibration. Since the actual amplitude of vibration is not measured, open loop controls permit over-driving of a vibrator, during which the actual amplitude of vibration exceeds the structurally-designed-for amplitude, and can damage a feeder.
It is not possible using open loop controls to accurately control the amplitude of vibration during the dribble feed portion of each weigh cycle. As noted above, the firing delay control voltages used to adjust the amplitude of vibration are non-linear, and the same voltage applied to different vibrators produces different amplitudes of vibration. A slight drop in the control voltage will not produce a corresponding drop in the amplitude of vibration. Moreover, in those cases where the product that is "in-flight" is insufficient to achieve the target weight, it is difficult using open loop control to provide an amplitude of vibration that is smaller than that of the dribble feed and for a brief time period to discharge a small amount of additional product into the bucket. The signal that is small enough to produce a smaller vibration in one pan of a multi-stage feeder or the various feeders of a combination may not cause any vibration in the other pans.
In a combination weighing machine, even a very experienced operator cannot monitor 12-15 feeders and pans using open-loop controls to adjust the amplitude of each in order to optimize the machine over an eight-hour shift. In a multiple stage feeder, as employed in either a single bucket or a combination weighing machine, it is virtually impossible to tune the pans so that the same voltage produces identical amplitudes, and thus identical feed rates in the pans. Moreover, in machines employing multiple stage feeders, altering the control voltages for the vibrators of a feeder in a uniform manner never reliably produces proportional alterations in the amplitude of vibration in each vibrator. Accordingly, product tends to bunch in one feeder while another feeder does not have enough product, and inconsistent feed rates result.
In order for a weighing device coupled to the bucket to accurately weigh the amount of discharged product in the bucket, any product that is discharged just before the cut off must first fall from the discharge end of the pan and settle in the weigh bucket. The weigh bucket is several inches, and generally at least 6-8 inches, and usually more than 12 inches, below the discharge end, and has sloped sides leading into the bottom of the bucket. The "in-flight" time for the discharged product to fall 6-8 inches is about 0.2 seconds. The product must then slide into and settle in the bucket, which can require up to several additional tenths of a second. Thus, the total time after cut off for product to travel into and settle in the bucket is between 0.3 and 0.5 seconds. The appropriate preselected times for bulk feed and dribble feed can to a degree be roughly anticipated for free-flowing products such as rice and shelled sunflower seeds, which produce consistent fill curves so long as depth of product in the pan and the amplitude of vibration of the pan is consistent. This is not the case for relatively large piece products such as candy bars, lollipops and frozen chicken parts, in which the "fill curve" shows discrete upward steps as pieces are discharged into a bucket.
In the case of relatively large piece products, it is very difficult to match the actual weight of product in a bucket or selected buckets, and the target weight. This results from the "step" nature of the fill curve. As the target weight is approached, individual pieces should be discharged into the bucket. However, as previously noted, it takes up to about 0.5 seconds after discharge before an accurate weight can be established. It is extremely time-consuming and inefficient to stop the vibrators each time a piece is discharged in order to weigh the new amount of product in the bucket. Consequently, the vibrators continue to run, and by the time that the target weight of product is detected in the bucket, too many additional pieces may be discharged. The addition of product into a bucket over and above the target weight, or "giveaways" is also inefficient.
As noted above, it is important to maintain a constant depth of product in the pans, and especially in the pan which discharges into the weigh bucket. The constant depth of product, in conjunction with a constant amplitude of vibration, ensures consistent, repeatable and efficient feed rates.
Known apparatus and methods for attempting to maintain a constant depth of product in feeder pans utilize "leveling trays" or optical sensors. The apparatus includes at least two pans arranged in series. The first pan discharges product into the bucket, and the second pan is loaded from a conveyor or an upstream pan, and discharges product into the first pan. The leveling tray or optical sensor is located at the feed end of the first pan, and determines whether product has backed-up in the first pan to a predetermined point at or near the feed end. If so, the tray or sensor signals for a temporary stop to the discharge of product from the second pan into the first pan. These known devices do not determine the weight of product in the first pan, which is a more accurate indication of depth of product in the first pan.
It is accordingly an object of the present invention to provide a method and apparatus for controlling a vibratory feeder in a weighing machine in which the amplitude of vibration is controlled in a closed-loop manner, to accurately and repeatably control the feed rate of product to be weighed.
It is another object of the present invention to provide a method and apparatus for controlling a multi-stage vibratory feeder in a weighing machine to establish and maintain a desired feed rate of product from a downstream pan by monitoring the weight of product in the downstream pan and controlling the amplitude of vibration of the preceding pan in a closed-loop manner.
It is a further object of the present invention to provide a method and apparatus for controlling a vibratory feeder in a weighing machine to rapidly and precisely ascertain the amount of product in the bucket, in order to decrease the time required to complete each feed cycle.